The present invention relates to a light emitting device including first and second electrode layers and a light emitting layer disposed therebetween, a lighting device including the light emitting device as a light source and a display device.
A light emitting device including a light emitting layer disposed between two electrodes can be made thin and is accordingly applied to a display device using the light emitting devices as pixels and a plate-like or sheet-like lighting device. Such a light emitting device uses inorganic or organic material as material of the light emitting layer. As the light emitting device using inorganic material as the light emitting layer, there are an electro-luminescence device and a light emitting diode device. Further, the light emitting device using organic material as the light emitting layer is named an organic electro-luminescence device or an organic light emitting diode device.
The organic light emitting diode device can be made to have a large area of the light emitting layer relatively easily by vapor deposition or using an ink jet method or a transfer printing method and can be operated at a low voltage. Accordingly, it is suitable for a thin and plate-like or sheet-like lighting device, particularly the backlight of a liquid crystal display device or the like.
These light emitting devices convert electric energy into light and since electric energy that does not contribute to emission of light is converted into heat, temperature of the light emitting device is raised during lighting.
The light emitting device has the problem that the life time thereof may shortened due to deterioration of the device in higher temperature or the light emission efficiency thereof is reduced. When the light emitting device is utilized as a light source of a lighting device, the lighting time thereof tends to be longer as compared with the case where the light emitting devices are utilized as pixels of a display device. Accordingly, the problem caused by the raised temperature of the light emitting device is apt to occur in the light emitting device.
In order to suppress the problem caused by the raised temperature of the light emitting device, it is preferable to dissipate the heat generated by the light emitting device as effectively as possible. To this end, it is preferable to select material having the high thermal conductivity as a substrate on which the light emitting device is formed.
As glass forming the substrate having the high thermal conductivity, there is sapphire glass, while it is preferable to use a substrate made of graphite or metal in order to realize the higher thermal conductivity. However, the substrate made of graphite or metal is generally opaque to visible light. When the substrate made of such opaque material is used, it is necessary to construct the light emitting device in which light is taken out from the opposite side to the substrate on which the light emitting device is formed, in order to utilize the light emitted from the light emitting device effectively. Such structure is named light out-coupling from top structure and the light emitting device having the light out-coupling from top structure is named a top emission type light emitting device.
An organic electro-luminescence device having the top emission type light emitting device formed on a metal substrate is disclosed in JP-A-2002-15859.